Gaseous-flow, discharge display device with an array of hollow cathodes

ABSTRACT

An electrical display device employing an array of electrically conductive dot cathodes and an anode or anodes for cooperating therewith to define an array of glow discharge paths, the dimensions and relative locations of the cathodes and anodes and pressure and composition of the atmosphere being such that only a negative flow discharge is produced and the positive column is suppressed.

United States Patent Allen et a1.

GASEOUS-FLOW, DISCHARGE DISPLAY DEVICE WITH AN ARRAY OF HOLLOW CATHODESlnventors: David Henry Odhams Allen, East Grinstead, Sussex; RaymondFrederick Hall, Crawley, Sussex; George Frederick Weston, Merstham,Surray, all of England Assignee: U.S. Philips Corporation, New

York,N.Y.

Filed: Jan. 22, 1971 Appl. No.: 108,965

Related U.S. Application Data Continuation of Ser. No. 766,525, Oct. 10,1968, abandoned.

Foreign Application Priority Data Aug. 29, 1968 Great Britain..46,406/68 U.S. Cl. ..313/209, 313/210, 313/217,

313/220, 315/169 TV Int. Cl. ..H0lj 61/06, H01 j 61/64 Field ofSearch.....315/l69 R, 169 TV; 313/210, 313/220, 109.5, 217, 209

[15] [451 Oct. 31, 1972 [56] References Cited UNITED STATES PATENTS2,848,638 8/1958 Smith ..3l3/2l7 X 3,042,823 7/ 1962 Willard ..315/169TV 3,206,638 9/ 1965 Moore ..315/169 TV 3,260,880 7/1966 Kupsky..313/210 X 3,334,269 8/1967 Ll-levreux ..315/169 TV 3,346,759 10/1967l-lardwick ..313/l09.5 3,042,823 7/1962 Willard ..315/169 X 3,206,6389/1965 Moore ..315/161 3,334,269 8/1967 LHevreux ..315/58 PrimaryExaminerPalmer C. Demeo Attorney-Frank R. Trifari [5 7] ABSTRACT Anelectrical display device employing an array of electrically conductivedot cathodes and an anode or anodes for cooperating therewith to definean array of glow discharge paths, the dimensions and relative locationsof the cathodes and anodes and pressure and composition of theatmosphere being such that only a negative flow discharge is producedand the positive column is suppressed.

2 Claims, 7 Drawing Figures minno s] m2 SHEET 1 BF 2 FIGI.

m m E V W.

F E D F HA DAVIDH.O. ALL

GEORGE w s'row BY RAYMON H7 AGENT I P'A'TE'N'TEDum 31 I972 SHEET 2 OF 2INVENTOR.

A VID H. O. A LLt N $5 BYD GASEOUS-FLOW, DISCHARGE DISPLAY DEVICE WITHAN ARRAY F HOLLOW CATI-IODES This application is a streamlinecontinuation of Ser. No. 766,525, filed Oct. 10, 1968, and nowabandoned.

This invention relates to electrical display devices. Such devices maybe used for displaying simple patterns such as diagrams, numerals, wordsor the like which do not require half-tone capabilities.

During the past few years an increasing amount of effort has beendevoted to flat display systems of one kind or another. Some suchsystems use electroluminescence as the light-emitting phenomenon. Inthese systems the present state of the art does not permit a brightnessin excess of a few tens of ft. L. This is not sufficient for daylightviewing purposes as required in the majority of the possibleapplications.

For this reason it has been proposed to employ a twodimensional array ofcold-cathode glow-discharge cells as the light sources for theindividual elements for the display. An early example of such anarrangement is described in US. Pat. Nos. 2,991,394 and 2,965,801

and illustrated in FIGS. and 11 thereof.

A more recent example is reported in Electronics News of July 26th 1965.In this proposal the individual cells consist of short gasdischarge-spaces arranged in the form of a two-dimensional matrix ofsmall apertures in a sheet of insulating material placed between twoelectrode systems of parallel wires or semi-transparent conductivestrips in a cross-bar arrangement. Light emission can be caused to arisefrom any element by passing a gas discharge through a hole in theinsulating matrix at the cross-over point of the two correspondingelectrode strips.

A difficulty with gas discharges of this type is the sputtering whichtakes place at the cathode, due to bombardment with positive ions. Thiscan severely limit the life of the device, and especiallysemi-transparent electrodes are very vulnerable in this respect.

Recent work has indicated that this problem can be successfully overcomeby the use of an array of hollow cathode discharge elements where eachcathode is recessed in a slab of insulating material. Arrangements ofthis character are described in US. Pat. No. 3,465,194.

According to the present invention an electrical display devicecomprises an envelope containing an array of separate cathode assemblieseach of which comprises an electrically conductive cathode situatedwithin a cavity which is open at one end and whose wall is electricallyinsulating, the envelope also containing at least one anode facing saidends for co-operating with said cathodes to define an array ofglow-discharge paths which extend through a gas atmosphere, a gapcontaining said atmosphere being present between said ends and theanode, the dimensions and relative locations of said cathodes and saidanode and the pressure and composition of said atmosphere being suchthat with the application of an operating potential across each pathnegative-glow optical radiation in the resultant glow-discharge isgenerated and the positive column part of the discharge is substantiallysup pressed.

Embodiments of the invention will now be described, by way of example,with reference to FIGS. 1 and 2 of the accompanying drawings in whichFIG. 1 shows an embodiment of an alpha-numeric indicator tube and FIG. 2shows a modification to the embodiment of FIG. 1.

In the drawings FIG. 1 shows an exploded view of an alpha-numericindicator tube. The parts denoted by reference letters a to f are:

FIG. 1a viewing window.

FIG. 1b side wall of envelope.

FIG. 10 anode.

- FIG. 1d matrix of holes in an electrically insulating plate.

FIG. 1e spacer for anode.

FIG. 1f cathode matrix.

The cathode matrix of FIG. 1f comprises an array of separate cathodes 1provided on an electrically insulating substrate 2. These cathodes arebrought out to terminals 3 for connection to an external source (notshown) of operating potential. The substrate 2 may be made of glass orceramic material and the array of cathodes 1 together with theconnections therefrom to the terminals 3 may be vacuum-depositedthereon. Preferably, however, the array of cathodes 1 together with theterminals 3 and the connections from the cathodes to these terminals maybe obtained by etching a sheet of metal, for example nickel-iron orchromeiron, so that the desired configuration is formed.

The resulting configuration may then be moulded into the surface of thesubstrate 2 by holding it in contact therewith and forming at a hightemperature, the connectors from the cathodes to the terminals 3 beingsubsequently bent round the edges of the substrate. In the sameoperation the spacer of FIG. 1e may be moulded onto. the top surface ofthe substrate and the cathode array.

The insulating plate of FIG. 1e may be made, for ex-v ample, of glass orceramic material. It is provided with a matrix of holes extending fromone major face thereof to the other and in positions such that theycoincide with the array of cathodes 1 when the insulating plate isplaced in contact with the substrate assembly l, 2, 3. The insulatingplate is of such size that it fits snuglyin the aperture provided in thespacer of FIG. 1e and rests in contact with the top surface of thesubstrate 2 provided with the cathodes.

The spacer of FIG. la is of a greater depth than the thickness of theinsulating plate 5 and thus provides a ridge around the edge of theinsulating plate 5 when assembled. On this ridge rests an opticallytransmissive electrically conductive anode plate 6 shown in FIG. 1e sothat the plate 6 is spaced from the plate 5. The anode 6 may be in theform of a perforated metal plate, the perforations coinciding with theapertures 4 when assembled to enable it to be optically transmissive andhence to enable optical radiation generated in the apertures 4 to beobserved through the top face of the anode. As an alternative the anode6 may comprise a fine wire mesh or, as a further alternative, it maycomprise a layer of optically transparent tin oxide deposited on theunderside of an optically transparent substrate (not shown).

Side-walls 7 and an optically transparent viewing window 8 complete theassembly, these being sealed together and to the top surface of spacer1e in a vacuum-tight manner so that the electrode assembly is enclosedwithin an envelope. After assembly the envelope may be evacuated througha pump stem 9 which may be subsequently sealed close to where it joinsthe side walls 7 and removed. r

In a practical embodiment the holes 4 were 1 mm. in

diameter spaced 2 mm. between their centers The spacing between the topsurface of the plate 5 and the bottom surface of the anode 6 wereapproximately 0.1 mm. The cathodes 1 and apertures in the anode 6 were 1mm. in diameter and again spaced 2 nun. between their centers so as toline up with the apertures in the plate 5. The envelope was evacuatedand then filled with pure neon or neon containing 2 percent argon to apressure of 150 Torr. These dimensions and pressure were such that theapplication of an operating potential difference of at least 280V forNeon and 160V for Ne- Ar between the anode and a given cathode orcathodes resulted in a glow-discharge in the discharge path or pathsdefined by the anode and this cathode or cathodes, which discharge gaverise to optical radiation from only the negative-glow portion of thedischarge, the positive column portion thereof being completelysuppressed. This radiation could be observed through the correspondinghole or holes in the anode 6, and the application of the operatingpotential to selected cathodes resulted in the production of a patterndetermined by this selection, which pattern was observable through thewindow 8. By suitable selection of the cathodes energized it was foundpossible to display successively a pattern making up each of the digitsof the decimal system, the cathodes being located in a 7 X 5 arrayj ltwas also found to be possible to display letters and other symbols.

FIG. 2 shows a possible alternative construction for thesubstrate/cathode/terminal assembly of FIG. 1f. In the substrateassembly the substrate 2 is pierced by metal wires 10 which are sealedinto the substrate 2. These wires may be again made of nickel-iron orchrome-iron and they terminate flush with the top surface of thesubstrate 2 to form the cathode 1. Their other ends may be used asterminals for connection to external circuitry. In both embodimentsnickel-iron or chrome-iron is preferred for the cathode material becausethis material may be easily sealed to a soft glass.

In a further embodiment a substrate assembly similar to that shown inFIG. 2 may be used but the tops of the wires 10 may be etched so thatthe wires 10 terminate some distance below the top surface of thesubstrate 2. In this way the matrix of FIG. 1d may be effectively formedintegrally with the substrate 2, this rendering the provision of aseparate matrix plate 5 unnecessary. In this particular embodiment anarray of cathodes will be formed similar to those described in theaforesaid co-pending British Pat. Application No. 44937/66.

The constructions described above with reference to FIGS. 1 and 2 havebeen found to be prone to electrical leakage across the interfacebetween the plate 5 and the substrate assembly 1, 2, 3, and hence toactivation of discharge from cathodes which are not directly energized.

This disadvantage can be overcome by moulding the plate 5 (FIG. 1d), thespacer of FIG. 1e and the substrate 2 (FIG. 1 in one piece around thecathodes 1 and the connections to the terminals 3 (FIG. 1]). Assemblymay also be facilitated by providing the window 8 and the side wall F ofthe envelope in one piece. A construction made in this way wasmanufactured as follows:

The array of cathodes 1 of FIG. 1, together with the terminals 3 and theconnection from the cathodes to these terminals was initially etchedfrom a flat sheet of metal available under the Registered Trade MarkKovar, portions of the sheet mechanically interconnecting the ends ofthe terminals 3 being left in situ for the moment in order to hold thecathodes l in their correct spaced relationship.

The resulting array was then placed in a mould with the terminals 3protruding from the side thereof. The interior of the mould had a flatbottom surrounded by a circumferential groove about 0.25 mm. deep. Anarray of vertical posts extended from the flat bottom. These posts hadcross-sections and relative positions identical with those of the arrayof cavities (similar to 4; FIG. 1d) which it was desired to form in theelectrical insulator. The tops of these posts terminated at individualcathodes 1. The interior of the other half of the mould had the shape ofthe substrate 2 (FIG. 1]). After the mould was closed it was filled withglass material available under the Registered Trade Mark F usite K andheld at 1,000 C in an inert gas atmosphere for 1 hour. This particularglass was found not to wet the cathode material when molten. On coolinga construction similar to a combination of the components shown in FIG.1d, 10 and 1 f was obtained after the leads 3 had been separated byremoving the portions of the sheet of cathode material which hadpreviously connected them.

An anode similar to that described with reference to FIG. 10 was thenplaced opposite the open ends of the cavities, resting on thecircumferential ridge formed by the mould groove on that surface of thenow solidified glass into which the cavities opened. Over this was thenplaced an envelope portion similar to a combination of parts 7 and 8 inFIG. 1. This portion was made up of material available under theRegistered Trade Mark Kodial and was sealed to the substrate 2 by meansof a solder glass. The envelope thus formed was filled with neoncontaining 2 percent argon at a total pressure of Torr.

The resulting display device was then aged by applying an operatingpotential of Volts or greater between each cathode and the anode andcontinuing this application until sputtering from the cathodes due tothe resulting electric discharge caused an appreciable amount of cathodematerial to be deposited on the cavity walls, the cathodes being ofsufiicient size so that this cathode material contacted the cathodeproper. Such sputtering inevitably occurs during operation and the agingwas carried out in order that the completed device should havepredictable characteristics which would not then change appreciably withaging. It should be noted that the anode is spaced from the open ends ofthe cavities so that this sputtered material cannot form a short circuitbetween cathode and anode. The display device was then ready for use andhad the following dimensions and characteristics.

Cavity cross-section: square of 1 mm side Cavity depth: 1 mm Cavityspacing: 2 mm between centers in a 7 X 5 array It may be advantageous toform a pattern of grooves between the open ends of each cavity in orderto increase the electrical leakage path length between the open end ofeach cavity and the next to provide additional protection againstleakage across this path.

A similar device to this and having substantially equivalent propertieswas also obtained by replacing the anode 6 of FIG. 1 by a mesh-likeanode formed by mechanically expanding a plate of the material availableunder the Registered Trade Mark Kovar.to such an extent that it had asatisfactory overall optical transmission.

What we claiin is:

1. An electrical display device comprising an envelope, an insulatingbody having a plurality of cavities therein disposed within theenvelope, an electrically conductive cathode at one end of each cavity,the walls of said cavities having a layer of cathode material thereon incontact with said cathodes, electrical supply conductors for eachcathode, said cathodes and the end portions of said conductors whichadjoin said cathode lying substantially in one plane, at least one anodewithin said envelope facing said cathodes and spaced from the edges ofsaid cavities, an ionizable gas filling in said envelope, said anode andsaid cathodes defining an array of glow discharge paths which extendthrough the gas, the dimensions and relative locations of said cathodesand said anode and the pressure and composition of said atmosphere beingsuch that with the application of an operating potential across eachpath nega tive-glow optical radiation in the resultant glow discharge isgenerated and is retained within said cavities and the positive columnpart of the discharge is suppressed.

- 2. A device as claimed in claim 1 wherein said body is substantiallyplate-shaped and the anode comprises an optically transmissiveelectrically conductive plate situated adjacent to major face of saidbody'and spaced therefrom.

1. An electrical display device comprising an envelope, an insulatingbody having a plurality of cavities therein disposed within theenvelope, an electrically conductive cathode at one end of each cavity,the walls of said cavities having a layer of cathode material thereon incontact with said cathodes, electrical supply Conductors for eachcathode, said cathodes and the end portions of said conductors whichadjoin said cathode lying substantially in one plane, at least one anodewithin said envelope facing said cathodes and spaced from the edges ofsaid cavities, an ionizable gas filling in said envelope, said anode andsaid cathodes defining an array of glow discharge paths which extendthrough the gas, the dimensions and relative locations of said cathodesand said anode and the pressure and composition of said atmosphere beingsuch that with the application of an operating potential across eachpath negative-glow optical radiation in the resultant glow discharge isgenerated and is retained within said cavities and the positive columnpart of the discharge is suppressed.
 2. A device as claimed in claim 1wherein said body is substantially plate-shaped and the anode comprisesan optically transmissive electrically conductive plate situatedadjacent a major face of said body and spaced therefrom.